The loudspeaker, a major element of an audio system, radiates acoustic power into the air with resultant waves equivalent in form to an electrical input. The direct radiator type loudspeaker, almost universally used for audio systems, is deficient for reproducing low frequency sound because of its low radiation mechanical resistance. In addition, the 180 degree phase relationship between waves radiated from front and rear radiator surfaces of the direct radiator loudspeaker reduces speaker efficiency and causes distortion. Currently, speaker cones are most frequently used as radiators in direct radiating type loudspeakers.
Radiation mechanical resistance can be increased by increasing speaker radiator size, however, larger radiators add cost and may not be practical within the available space. Currently, the most common method for increasing radiation mechanical resistance is the mounting of the speaker in a cabinet, alternately referred to as an enclosure, whereby the rear surface of the speaker radiator is loaded by a volume of air. Enclosures may be designed to absorb rear radiation from the speaker radiator in part or in total, to augment reproduction of certain frequencies by phasing front and rear speaker waves or a combination thereof.
A variety of enclosure designs are available for improving performance of the direct radiator loudspeaker, the most common being the "acoustic suspension" and "bass reflex" enclosures. The "acoustic suspension" enclosure derives its name from the manner of increasing radiation mechanical resistance whereby the acoustic capacitance of a confined volume of air in a non-vented enclosure is used for supplementing the speaker radiator restoring force normally supplied by the cone suspension. In actual practice, "acoustic suspension" systems tend to be compact but inefficient and tend to distort from excessive one sided loading of the speaker radiator.
The popular "brass reflex" enclosure by phasing and coupling waves radiated from the front and back surfaces of a speaker radiator is efficient for reproducing bass frequencies. However, "bass reflex" enclosures tend to be of large size, have poor bass transient response and distort from cabinet resonance.
Radiated energy from the speaker rear cone surface could be totally dissipated by transmitting waves from the rear cone surface down a transmission line of infinite length. Since this is impractical, enclosures based on the long transmission principle have been developed. A long folded transmission line enclosure, the "acoustic labyrinth," has been further improved by lining the surfaces of the transmission line with acoustic absorption fiber materials. See Olney U.S. Pat. No. 2,031,500 which teaches a folded transmission line enclosure having acoustic absorption linings.
The transmission line length of the "acoustic labyrinth" enclosure has been reduced to 8 feet by filling the line with loosely packed wool of density range one pound per 2 to 3 cubic feet. The inventor of this improvement found higher wool packing densities and shorter transmission lines with his system to be unsatisfactory. See A. R. Bailey, A Non-resonant Loudspeaker Enclosure Design, Wireless World, October, 1965, and A. R. Bailey, The Transmission Line Loudspeaker Enclosure, Wireless World, May, 1972.